Gearboxes can be individually adapted to different types of vehicles with the aid of gearbox control systems. In doing so, the gearbox control system makes possible the integration of functions that are relevant to comfort, safety, and service. The shifting comfort is generally in the foreground, even though it is directly felt by the driver. Other important functions concern the shift point control, safety, and diagnosis. Shifting comfort is still quite important. An excellent shifting comfort, which remains constant during the entire service life of the gearbox, is the main duty of the gearbox control system.
The shifting comfort is influenced by a variety of parameters which are described in the ATZ Automobiltechnische Zeitschrift 93 (1991). By means of an optimal combination of friction elements, such as multi-plate clutches and brake band and the corresponding gearbox oil, a constant friction value level can be achieved. The shifting pressure of the multi-plate clutches or of the brake band is controlled in dependence upon parameters such as gear stage, engine speed, throttle flap angle for thrust-traction recognition, gearbox output speed, engine speed, gearbox temperature, and engine temperature in such a way that a uniform shifting operation always takes place. Furthermore, an adaptive pressure control takes place; that is, the deviations from specific parameters are automatically corrected during the operation time of the gearbox.
For this reason, the response times or gliding times are permanently measured and average response times are determined from several shifting operations. If there is a deviation of the average response time from desired response times dependent from the load and engine speed, the clutch pressure is changed until the desired response time is reached.
In this solution, which is known from the state of the art, it is not possible to detect, however, and correct systematic errors and long-term changes, since gearbox transmission errors cannot be detected.
In EP 0 531 567 A1 a gearbox control system for a motor vehicle drive with an automatic gearbox is described, which makes it possible to adapt shifting characteristic lines. The control system has a characteristic line storage in which the shifting points of the gearbox are stored, and additional devices can be modified by means of which the shifting points in dependence upon the driving parameters. Based on an evaluation of the output speed of the gearbox, carried out at successive time in intervals, the shifting arrangement generates an output signal by means of which the shifting characteristic lines can be adapted to the load condition of the motor vehicle. This control system does not make possible, however, a shifting pressure change and thus also the gearbox torque transmission behavior cannot be adapted to the long-term changes.
It is an object of the invention to present a gearbox control sys tem which can detect gearbox transmission errors and thus can compensate for systematic errors and long-term changes.
The basic object of the invention is attained by means of a gearbox control system of the type described above.